METHOD FOR CLEANING A GAS FLOW

Abstract: In the cleaning of a humid gas flow containing gaseous impurities, the gas flow is dehumidified by cooling. The cooled gas flow is reheated and then conducted through a device (7') for the adsorption of gaseous impurities. A desorption-gas flow is heated so as to form a hot-gas flow, which is conducted through the adsorption device (7') in order to clean the latter. The contaminated hot-gas flow is conducted to a combustion device (10') in order to be burnt. Hot combustion gases are removed from the combustion device (10') in order to take part in at least one of the heating processes by being introduced into and mixed with the cooled gas flow and the desorption-gas flow.

Description: METHOD FOR CLEANING A GAS FLOW This invention relates to a method for cleaning a humid gas flow containing gaseous impurities, such as an exhaust-air flow containing volatile solvents and origi¬ nating from a spray booth in a painting plant, in which method the gas flow is dehumidified by cooling, the cool¬ ed gas flow is reheated to subsequently be conducted through a device for the adsorption of gaseous impuri¬ ties, a desorption-gas flow is heated to form a hot-gas flow, which is conducted through the adsorption device in order to clean the latter and entrain impurities accumu¬ lated therein, the contaminated hot-gas flow is conducted to a combustion device in order to be burnt, and hot com¬ bustion gases are removed from the combustion device in order to take part in at least one of the heating pro- cesses. The above method is used in a painting plant for cleaning an exhaust-air flow containing paint particles and volatile solvents and originating from a spray booth in the plant. In this prior-art method, the gas flow is conducted through a dust separator, where particulate impurities are separated from the gas flow, which is cooled when leaving the dust separator. Part of the gas flow leaving the dust separator is conducted, as cooling- gas flow, through the adsorption device in order to cool those parts of this device that have been heated by the hot-gas flow, whereupon the cooling-gas flow is used as desorption-gas flow. Hot combustion gases from the combustion device are conducted through a heat exchanger in order there to emit heat to the desorption-gas flow, which also is conducted through the heat exchanger. Further, a heat exchanger is used for reheating the cooled gas flow leaving the dust separator. The object of the present invention is to provide a cleaning method in which the energy consumption is lower and there is less need of heat exchangers than in the above prior-art method, and which thus is less expensive to implement than the prior-art method. According to the invention, this object is attained by a method which is of the type stated by way of intro¬ duction and is characterised in that hot combustion gases from the combustion device are introduced into and mixed with the cooled gas flow and the desorption-gas flow. It will be appreciated that the two heat exchangers required for implementing the above prior-art method can be dispensed with when utilising the method according to the invention. The cooling-gas flow, which in the prior- art method is dimensioned to serve as hot-gas flow as well and thus is unnecessarily large, may, if used in a corresponding fashion when implementing the method according to the invention, be advantageously reduced to merely fulfil the cooling function, hot combustion gases being supplied to the desorption-gas flow so as to form the hot-gas flow. The invention will now be described in more detail with reference to the accompanying drawings, in which Fig. 1 schematically illustrates a plant which is intended for implementing the inventive method and in which use is made of an incinerator with a recuperative heat exchanger; and Fig. 2 schematically illustrates a plant which is intended for implementing the inventive method and in which use is made of an incinerator with a regenerative heat exchanger. In Figs 1 and 2, the encircled numerals at the dif¬ ferent flow arrows indicate a flow value for the respec¬ tive flows, this flow value bearing a relation to the air flow which prevails in a spray booth and whose flow value has been set at 100. The temperature of the respective flows is also indicated at some flow arrows. Both the plant illustrated in Fig. 1 and that illu¬ strated in Fig. 2 are used for cleaning the exhaust-air flow containing paint particles, volatile solvents and water vapour that leaves a spray booth 1 in a plant for painting vehicle bodies. Via an air conditioner 2, an air flow is blown into the spray booth 1 by means of a fan 3. The major part (about 90%) of the air blown into the spray booth 1 is air that is circulated in the system in a manner to be described in more detail below, whereas the remainder (about 10%) is taken from the surrounding atmosphere. The air is conditioned in the air conditioner 2 in such a manner that an air flow having a temperature of about 23°C and a relative humidity of about 60% is obtained in the spray booth 1. The air flow leaving the spray booth 1 and having a temperature of about 18°C is conducted through a dust separator 4, such as a wet electrostatic precipitator, where particulate impurities, such as paint particles, are separated from the air flow. When leaving the dust separator 4, the air flow is dehumidified by cooling in a cooler 5. The cooled air flow leaving the cooler 5 is, as will be described in more detail below, supplemented with an additional flow with which it is conducted, by means of a fan 6, through a device 7' (Fig. 1) or 7" (Fig. 2) for the adsorption of gaseous impurities, such as volatile solvents. The adsorption device 7' or 7", which is of known design, consists of a rotor composed of a plurality of segment-shaped adsorption elements containing a suitable adsorbing agent, which is zeolite in the rotor 7' shown in Fig. 1 and active carbon in the rotor 7" shown in Fig. 2. US-A-5,057,128, for instance, discloses a rotor of this type. The flow path through the rotor 7' or 7" is, by means of fixed channel walls (not shown), divided into three sectors, namely a gas-cleaning sector which covers most of the circular cross-sectional area of the rotor and through which is conducted the main part of the gas flow to be cleaned, and two minor sectors, of which one is a cooling sector through which a minor part of the gas flow to be cleaned is conducted in order to cool the adsorption elements, and the other is a desorption sector through which a hot-gas flow is conducted in order to clean the rotor and entrain impurities accumulated there¬ in. The cooling-air flow is regulated by means of a register 8. The rotor 7' or 7" rotates during use, so that the portion thereof situated opposite to the gas- cleaning sector of the flow path is gradually inserted into the desorption sector of the flow path so as to be cleaned, and then inserted into the cooling sector of the flow path in order to be cooled. The major part of the cleaned gas flow leaving the adsorption device or the rotor 7' or 7" is, as mentioned above, recycled to the air intake of the fan 3, whereas the remainder of the gas flow is discharged into the sur¬ rounding atmosphere. The hot-gas flow, which has a high solvent concen¬ tration when leaving the adsorption device 7' or 7", is conducted to a combustion device 10' or 10" by means of a fan 9. In the Example illustrated in Fig. 1, the combustion device 10' is an incinerator of the type described in e.g. DE-A1-30 41 269 and DE-A1-30 43 286 and has a corn- bustion chamber 11 and a recuperative heat exchanger 12 through which the heavily contaminated hot-gas flow is introduced into the combustion chamber 11. The combustion gases from the combustion chamber 11 are removed through the heat exchanger 12, where they emit heat to preheat the hot-gas flow, so that their temperature is reduced from about 730°C to about 350°C. A part (about 40%) of the thus-cooled combustion gases is conducted, via a flow-regulating register 13, to the desorption sector there to be mixed with the cooling- air flow from the adsorption device 7' and form, together therewith, the hot-gas flow. The cooling-air flow leaving the adsorption device 7' has a temperature of about 60°C and is mixed with combustion gases having a temperature of about 350°C in such proportions (6:4) that the hot-gas flow obtains a temperature of about 180°C. When leaving the adsorption device 7' , the hot-gas flow has a tempera- ture of about 90°C. A part (about 30%) of the combustion gases cooled in the heat exchanger 12 forms the above-mentioned additional flow and is conducted to the outlet of the cooler 5 to be mixed with the cooled air flow leaving this device. The cooled air flow leaving the cooler 5 has a temperature of about 15°C and is mixed with combustion gases having a temperature of about 350°C in such propor¬ tions (100:3) that a gas flow having a temperature of about 25°C is obtained. The remainder (about 30%) of the combustion gases cooled in the heat exchanger 12 is conducted through a heat-recovery device 14 before being let out into the surrounding atmosphere. The plant illustrated in Fig. 2 differs from that illustrated in Fig. 1 in that active carbon, and not zeolite, is used in the adsorption device, as mentioned above, and in that an incinerator with a regenerative heat exchanger, and not a recuperative heat exchanger, is used as combustion device. The incinerator employed is of the type described in e.g. SE-A-8403330-7 and SE-A- 8802791-7. The hot-gas flow, which has a high solvent concen¬ tration when leaving the adsorption device 7", is con¬ ducted to the combustion device 10", i.e. the regene- rative incinerator, where combustion takes place. The combustion gases removed from the combustion device 10" have a temperature of about 206°C. A part (about 45%) of the combustion gases is conducted, via a register 13, to the desorption sector there to be mixed with the cooling-air flow from the adsorption device 7" and form therewith the hot-gas flow. The cooling-air flow leaving the adsorption device 7" has a temperature of about 50°C and is mixed with combustion gases having a temperature of about 206°C in such proportions (5.5:4.5) that the hot-gas flow obtains a temperature of about 120°C. When leaving the adsorption device 7", the hot-gas flow has a temperature of about 70°C. The remainder (about 55%) of the combustion gases is conducted to the outlet of the cooler 5 to be mixed with the cooled air flow leaving this device. This air flow, which has a temperature of about 15°C as in the plant illustrated in Fig. 1, is mixed with combustion gases having a temperature of about 206°C in such proportions (100:5.5) that a gas flow hav¬ ing a temperature of about 25°C is obtained. In the plant illustrated in Fig. 2, the energy contained in the com¬ bustion gases is utilised to the full, for which reason there is no need of any special heat-recovery device.

Claims: CLAIMS 1. A method for cleaning a humid gas flow containing gaseous impurities, such as an exhaust-air flow contain¬ ing volatile solvents and originating from a spray booth (1) in a painting plant, in which method the gas flow is dehumidified by cooling, the cooled gas flow is reheated to subsequently be conducted through a device (7' ; 7" ) for the adsorption of gaseous impurities, a desorption- gas flow is heated to form a hot-gas flow, which is conducted through the adsorption device (7'; 7") in order to clean the latter and entrain impurities accumulated therein, the contaminated hot-gas flow is conducted to a combustion device (10'; 10") in order to be burnt, and hot combustion gases are removed from the combustion device (10'; 10") in order to take part in at least one of the heating processes, c h a r a c t e r i s e d in that hot combustion gases from the combustion device (10', 10") are introduced into and mixed with the cooled gas flow and the desorption-gas flow. 2. A method as set forth in claim 1, in which part of the gas flow leaving the adsorption device (7'; 7") is used as desorption-gas flow, c h a r a c t e r i s e d in that the hot-gas flow is formed by supplying hot com¬ bustion gases from the combustion device (10'; 10") to the desorption-gas flow. 3. A method as set forth in claim 1, in which part of the cooled and subsequently reheated gas flow is con- ducted through the adsorption device (7'; 7") as a cool¬ ing-gas flow for cooling those parts of the adsorption device that have been heated by the hot-gas flow, where¬ upon the cooling-gas flow is used as desorption-gas flow, c h a r a c t e r i s e d in that the hot-gas flow is formed by supplying hot combustion gases from the com¬ bustion device (10'; 10") to the desorption-gas flow. 4. A method as set forth in claim 2 or 3, c h a r a c t e r i s e d in that hot combustion gases from the combustion device (10'; 10") are supplied to the cooled gas flow before this is conducted through the adsorption device (7'; 7").

Inventors: ENEROTH JAN-MATS, PERSSON ANDERS, ENEROTH, JAN-MATS, PERSSON, ANDERS
Applicants: ENEROTH JAN MATS, FLAEKT AB, PERSSON ANDERS, ABB FLAEKT AB, ENEROTH, JAN-MATS, PERSSON, ANDERS
Application year: 1994
Main classification: B01D53
IPC codes: B01D53, F23G7